EIUWC has recently received phone calls and emails concerning questions about the radar image we obtain from NWS Lincoln. Below is a summarization of those particular enquiries.

"The radar images shows blue colors throughout the area but there is no rain here?"

Sometimes on a clear beautiful day, a radar image may display small areas of blue blotches around the region. These blue pixels do not necessarily indicate that rain will ruin your day. The radar is receiving false echo returns from fixed ground objects such as buildings and hills near the radar site. Other false returns are caused by days of extreme haze and particulates in the air.

The radar image above from NWS Lincoln shows a great deal of ground clutter and high amounts of moisture, which some think that there is precipitation occurring. When the image was acquired there was no precipitation of any kind throughout the region.

"Do orange and red colors in a radar image indicates tornadoes?"

The various colors on a radar image (base reflectivity) indicate the intensity of a precipitation event. Blue to Green usually indicate light precipitation, while yellow, orange, and red display areas where the precipitation is extremely heavy. Red colors are often associated with severe situations by the general public but should not always be confused with a tornado siting. To determine if a tornado is present, the velocity radar image is used that displays the wind speed and direction.

"If I want to see if there is a tornado somewhere in the region, how do I see it using a velocity image?"

A velocity radar image shows the character of the wind around the radar site. Purple to Red pixels in a velocity image indicate that the winds are flowing toward the radar site, while blue to green pixels show the winds flowing away from the radar. When bright red and green pixels are isolated together, it indicates that the winds are rotating in that area. It is important to get confirmation concerning the possibility of a tornado by listening to the severe weather broadcast on NOAA Radios. Just because you might see the rotation signature does not always mean there is a tornado on the ground. Trained Storm Spotters are always called by NWS Meteorologists to confirm what is happening in the area of the signature.

"The radar shows rain over my area but it is still dry at the surface."

When a radar indicates an area of rain but it is not raining at the surface, the precipitation is evaporating before it reaches the surface. Sometimes you can see rain falling from a cloud then it disappears. This is called virga (rain that evaporates as it falls out of a cloud). This occurs when the surface dew point depression (Temperature - Dew Point) is extremely high. Although the dew point depression may be very low near the base of a cloud, the rain can evaporate quickly near the surface if the air is dry. If the dew point depression is low between the cloud and ground, then rain will completely reach the surface.

Do you have more questions about radar images? Send us an email, cdcraig@eiu.edu.